The present invention relates to a golf ball material which is suitable for use as a cover material in golf balls, has good flow properties and moldability, and endows the golf ball with an excellent rebound resilience and durability even when the material hardness is low. The invention also relates to a method of preparing such a golf ball material.
Ionomers are conventionally used in golf balls. Approaches being carried out recently to achieve a higher ball rebound in particular include methods that involve blending ionomers together, methods in which other thermoplastic resins and additives are blended together with an ionomer, and methods that increase the degree of neutralization of an ionomer itself.
Among methods that involve blending ionomers together, a number of techniques which use two types of ionomers having different weight-average molecular weights have been proposed. For example, U.S. Pat. No. 7,462,113 discloses the use in a cover material of a ternary ionomer having a weight-average molecular weight of from 80,000 to 500,000 in combination with a ternary ionomer having a weight-average molecular weight of from 2,000 to 30,000. Also, U.S. Pat. Nos. 7,273,903 and 7,488,778 describe blending together, as a cover material: a ternary ionomer having a weight-average molecular weight of from 80,000 to 500,000, a binary ionomer having a weight-average molecular weight of from 2,000 to 30,000, and an optional thermoplastic elastomer.
However, the above proposed ionomer blending techniques all use resins (ionomers) that have been neutralized beforehand with metal ions, resulting in a lower degree of freedom in blend design—such as the types of metal ions, the blending amounts, and the degrees of neutralization—which has made it difficult to achieve the desired properties.
When use is made of a highly neutralized material in which the degree of ionomer neutralization has been increased, such a material tends to have a higher hardness, as a result of which the rebound resilience of the material has a tendency to increase. Conversely, efforts to lower the hardness of a highly neutralized ionomer lead to a diminution of the high rebound resilience characteristic of highly neutralized ionomeric materials, which poses a challenge. Recently, various art has been disclosed which, by softening the cover material used in a golf ball, improves the feel of the ball upon impact and also increases the spin rate on approach shots, thus enhancing the controllability. However, when the cover is soft, the ball has a poor rebound and a spin rate-lowering effect cannot be achieved, as a result of which the ball tends to travel a shorter distance.
Accordingly, there exists a desire for, in cases where a highly neutralized ionomeric material with the advantage of having a high degree of freedom in blend design is used as the cover material, a golf ball material which enables a high golf ball rebound and durability to be retained even when the material hardness is low.